Strainer devices of the above related sort can be divided into two main types, to wit a first type being equipped with means for back-flushing of the strainer housing, and a second type which completely lacks such means. The first mentioned strainer devices are advantageous from a safety point of view, as long as they permit a cleaning of the apertured strainer walls of the housing at locations where these walls would unintentionally be clogged or blocked by fibres or other impurities circulating in the containment. This is effected by feeding clean wash-water into the strainer housing through a special feeding conduit with a pumping unit being connected thereto, the latter being actuable when necessary.
However, an important inconvenience of such strainer arrangements is that the pumping unit as well as the special wash-water conduit are costly to produce and install. Moreover, they require considerable space in the area outside the strainer housing. It is true that the other type of strainer arrangements, i.e., those which completely lack back-flushing means, are comparatively inexpensive and space-saving, but they are limited in regard to the safety aspect, since they stop functioning if the holes in the strainer walls become clogged.
In case fibres cumulate on the outside of the apertured envelope surface on a strainer housing, the fibres will form a continuous, circumferential mat. With previously known strainer arrangements, considerable difficulties have been encountered when detaching this fibre mat in connection with a backflushing. The washwater which is brought to flow from the inside in a direction radially outwards through the perforations in the strainer wall, does not bring about any complete and immediate release of the fibre mat; initially the water flow merely stretches the mat during simultaneous breaking up of the fibre structure. The removal of the mat is thus accomplished in such a way that individual fibres are successively released and removed from the mat. It is only after considerable hydromechanical action that the mat starts getting weaker and is gradually divided into chunks that leave the strainer wall. In order to cope with these difficulties, it has recently been suggested to provide radially protruding wings on the outside of the strainer wall, which wings split a cumulated fibre mat into several, peripherally separate sections, each one of which easily detaches from the strainer wall in connection with a backflushing. This is disclosed in PCT/SE93/01042.
According to preferred embodiments of the present invention, radially protruding wings of the above mentioned type are foreseen, which are thus known per se.